1. Field of the Invention
The present invention relates to a projection exposure apparatus used in the lithography process for semiconductor devices, liquid display devices or the like, and more particularly to an alignment apparatus which can reduce an extent of alignment errors attributable to distortion caused by a projection system of the projection exposure apparatus.
2. Related Background Art
Recently, steppers mounting projection lenses of large numerical aperture thereon have been widely used as apparatus for printing a pattern of masks (reticles) on semiconductor wafers with resolution on the order of submicrons.
In such a stepper, a chip pattern (shot area) already formed on the wafer and a reticle pattern newly exposed in superposed relation must be aligned with each other at overall precision less than a fraction of the minimum line width. Of late, therefore, steppers mounting alignment apparatus (sensors) with an ability of higher precision thereon have been researched and developed for practical use.
In the future, it is believed that those steppers which will be primarily used in producing 4 Mbit D-RAMs and 16 Mbit D-RAMs will of necessity have a die-by-die exposure mode using the TTR (Through-The-Reticle) method in which a mark on the reticle and a mark in each shot area on the wafer are successively detected and aligned with each other, followed by printing.
While various techniques have so far been proposed to implement the TTR alignment method, most promising one is a different wavelength TTR alignment method in which the reticle mark and the wafer mark are simultaneously detected by using an illumination light different in wavelength from an exposure light. This type alignment method is advantageous in that because there will not occur a phenomenon for a resist layer on the wafer to strongly absorb the exposure light, the mark can be stably detected even for such wafers as having a resist impregnated with dyes or a multi-layered resist, and the resist in a mark area can be prevented from being exposed or sensitized upon illumination for alignment. Typical techniques (projection exposure apparatus) well known as to implement the different wavelength TTR alignment method are disclosed in U.S. Pat. Nos. 4,251,160, 4,269,505, 4,492,459 or 4,473,293.
In any of those typical projection exposure apparatus, however, an optical system for correcting chromatic aberration of the illumination light having the different wavelength for alignment is disposed between a reticle and a projection lens. Such a correction optical system serves to maintain the reticle mark and the wafer mark in focused relation to each other under the illumination light having the different wavelength, but has suffered from an intrinsic problem that stability is insufficient and precise reproducibility cannot be obtained in the alignment.
In the last several years, a method permitting the different wavelength TTR alignment without using such an correction optical system with less stability and reproducibility has been proposed in U.S. Pat. No. 4,880,310 or Japanese Patent Laid Open No. 63-283129 (corresponding to U.S. application Ser. No. 192,784 filed on May, 10, 1988). In an alignment system disclosed in the above reference, beams for illuminating the reticle mark and the wafer mark are simultaneously focused by a two-focusing element and an object lens on two planes, respectively. One plane is coincident with a pattern plane (mark plane) of the reticle, whereas the other plate is coincident with a wafer conjugate plane in a space away from the reticle pattern plane by a distance corresponding to the amount of axial (on-axis) chromatic aberration of the projection lens.
Adopting the above disclosed method eliminates the need of providing an optical element, other than the projection lens, in an optical path for alignment between the reticle and the wafer, and permits the TTR alignment as if the exposure light is used.
However, the latest projection lens is corrected in its various aberrations satisfactorily for only the exposure light, but exhibits both axial chromatic aberration and magnification chromatic aberration. Even if use of the two-focusing element succeeds in correcting the axial chromatic aberration, the magnification chromatic aberration cannot be always corrected satisfactorily, thereby requiring it to remove an alignment error (offset) attributable to the magnification chromatic aberration by some method. For this reason, a technique of coping with the magnification chromatic aberration (distortion) has been proposed in U.S. Pat. Nos. 4,780,913 and 4,856,905, by way of example. Of them, U.S. Pat. No. 4,780,913 discloses such a technique that a reference mark capable of exiting light rays at two wavelengths, i.e., the exposure light at one wavelength and the illumination light for alignment at a different wavelength, is provided on a wafer stage and moved on the image plane side of a projection lens to scan a retro-projected image of the reference mark on the reticle side for determining a position of the reticle mark, thereby preparing a distortion map in the view field of the projection lens beforehand. On the other hand, U.S. Pat. No. 4,856,905 discloses such a technique that the exposure light is introduced in the form of a beam to an illumination beam transmitting system (comprising a scanner mirror, two-focusing element, object lens, etc.) of a different wavelength TTR alignment system, allowing an illumination beam at the different wavelength and an illumination beam at the wavelength of the exposure light to be scanned simultaneously, whereby data of light information are photoelectrically detected from respective reference marks on the reticle mark and the wafer stage to determine a distortion at the alignment point based on differences in the amount of position deviation detected for each wavelength of the beam.
The aforementioned prior arts for coping with the magnification chromatic aberration are arranged so that both the illumination light for alignment and the illumination light for exposure pass through the same position in the view field of the projection lens. Specifically, in U.S. Pat. No. 4,780,913, two types of illumination light are required to be introduced to the rear side of the same reference mark through optical fibers or the like. In U.S. Pat. No. 4,856,905, the illumination light for exposure is required to be introduced to the different wavelength TTR alignment system.
That structure has suffered from problems as follows. An arrangement of the alignment optical system is complicated, which leads to a difficulty in manufacture that a severer level is required in the performance of constituent optical elements (particularly, achromatism). In addition, it is difficult to make a match between various conditions of the optical system (such as a sigma value, number of aperture and tele-centricity) under the illumination light for exposure and various conditions of the optical system under the illumination light for alignment, thus rendering it hard to know a precise distortion error enough for practical use. Moreover, an actual exposure apparatus (stepper) of even 1/5 reduction type is designed so as to accommodate an exposure of a wide field (view field) on the order of 15.times.15 mm to 20.times.20 mm. But, exposure areas of reticle patterns employed by stepper users are versatile in size, and the position of the alignment mark in the projection field is naturally changed variously depending on the size. Because the distortion amount of the projection lens with respect to an ideal lattice under the illumination light for exposure is also changed depending on change in the alignment position, there has been another problem that the difference between distortion characteristics under the illumination light for alignment and the illumination light for exposure, which has been determined at only the alignment position, is not enough for satisfactory correction, taking into account the fact that the reticle pattern must be superposed with the shot area over the entire wide field.
Further, the apparatus disclosed in the above cited U.S. Pat. No. 4,856,905 has a specific problem as follows. Where the illumination light for exposure and the illumination light for alignment are separated from each other depending on their ranges of wavelength by a dichroic mirror obliquely disposed above the reticle at an angle of 45.degree., if transmissivity (or reflectivity) of the dichroic mirror for the illumination light for alignment is set very high, the illumination light for exposure to be detected by the different wavelength TTR alignment system could not pass through (or be reflected by) the TTR alignment system in its large part, making is difficult to detect the mark. On the other hand, if wavelength characteristics of the dichroic mirror are selected to give some degree of transmissivity (or reflectivity) to the illumination light for exposure as well, the amount or intensity of light directing toward the reticle from an exposure light illuminating system during the exposure would now be reduced correspondingly.